An optical head device which is capable of reproducing and recording information from and in different types of an optical recording medium (optical disk) such as a CD and a DVD is, for example, structured such that a twin laser light source is used which is provided with a first laser beam emitting element and a second laser beam emitting element which emit laser beams with different wavelengths. When the twin laser light source is used, two laser beams are passed through an approximately common optical path. Therefore, since an optical path composite element is not required, the number of parts is reduced and thus cost can be reduced (see, for example, Japanese Patent Laid-Open No. 2003-272218 and Japanese Patent Laid-Open No. 2006-24276).
However, in an optical head device provided with a twin laser light source, when a focusing error signal is to be detected by an astigmatism method, there are the following problems.
When wavelengths are different, axial chromatic aberration of a lens is occurred. On the other hand, an optimal position of each of the optical elements is determined by its wavelength. Therefore, when a plurality of laser beam emitting elements is provided for respective wavelengths, defocus adjustments are required to be performed by optimizing positions of the light emitting elements and other optical elements for respective wavelengths. However, when a twin laser light source is used, since two laser beam emitting elements are integrated with each other, defocus adjustment by utilizing the above-mentioned method is unable to be performed.
Further, in order to enhance qualities of reproducing and recording, a detection lens is sometimes disposed before a light receiving element. However, when the twin laser light source is used, in addition to the problem of the above-mentioned axial chromatic aberration, two laser beam emitting elements are disposed apart about 110 μm from each other. Therefore, the optical axis of a return light beam of at least one of the laser beams is located at an off-axis position of the detection lens and thus curvature of image field is occurred.
Accordingly, when the twin laser light source is used, a difference between focus error offsets of the respective wavelengths becomes large and thus a servo-control is not performed satisfactory to cause signal quality to reduce.